Microchemical and microstructural investigations of degradation in asbestos-cement sheet

Crystal chemistry of cement-asbestos

“…As a matter of fact, chemical analyses and mineralogical phase compositions, reported in Tables 2 and 3, respectively, reflect both the complexity of the original CA matrix and the results of degradation processes. Similarly to other aged CA samples, calcite is the most abundant crystalline phase detected, being considered the main product of weathering of the cement matrix (Bassani et al 2007;Dias et al 2008;Laviano et al 2002;Sarkar et al 1987). The weight fraction of the amorphous fraction, usually not determined in XRPD analyses of hardened cement pastes and old cement based products, is quite variable, ranging from 9.5 to 66.6 wt%, and correlates inversely with carbonates.…”

“…It is hard to say whether calcium carbonate was used as mineral filler or not. Calcite has been detected in a qualitative XRPD of a sample of CA roof slate, described as unaltered (Sarkar et al 1987). However, secondary origin cannot be excluded, since it is known that, under ordinary conditions, carbonation of the cement matrix is a relatively fast process.…”

“…In a few cases, some less common crystalline phases were detected, see for example wollastonite in sample 14, or leucite in samples 15 and 17. In Figures 3c and 3d been extensively employed in materials undergoing autoclave processes (Bentur and Mindess 2007;Sarkar et al 1987). Substituting up to 40 wt% of OPC, it plays an important role during hydration because of its pozzolanic activity.…”

“…In practice, in aged materials, minor amounts of unreacted cement phases are still found, and, since CA matrix can suffer from deterioration like any other matrix of OPC-based pastes, concrete secondary products will be detected. As a matter of fact, CAs currently removed from the environment, have been in service for at least 20 yr, thus, severe deterioration may have occurred (Burdett 2007;Dias et al 2008;Laviano et al 2002;Sarkar et al 1987;Spurny et al 1989). Its extent will be a function of the original chemistry and mineralogy, age, conditions of use, local climate, as well as contact with aggressive fluids.…”

“…Their widespread distribution is obviously a matter of great concern as it has been long recognized that CA in open environment undergoes physical-chemical degradation which eventually leads to the release of breathable fibers (Babic 2006;Dias et al 2008;Spurny et al 1989). Motivated by human health concerns, a substantial effort was devoted to the detection and potential dispersion of asbestos minerals from ACM (Burdett 2007;Sarkar et al 1987;Spurny et al 1989). Since CA are the first fiber reinforced cement composites of modern times, the degradation of their mechanical properties with time have been extensively studied (Bentur and Mindess 2007), whereas their chemical and mineralogical characterization raised less scientific interest and has usually been limited to the study of few samples (Dias et al 2008;Laviano et al 2002).…”